Ferromagnetic metallic glasses have received much attention in recent years because of their exceptional magnetic properties which make them particularly suitable for use in the production of magnetic cores, especially cores for distribution transformers. However, the shape of metallic glasses products that can be produced in quantity by casting directly from the melt remains limited to thin ribbons (less than about 0.1 mm thick). Accordingly, magnetic cores produced from metallic glasses are primarily formed by winding continuous thin ribbon to form a spiral core.
Magnetic cores are, however, also produced from stacked layers of ferromagnetic material. Presently, cores for distribution transformer application are produced from silicon steel plates usually about 0.007-0.014 in (0.18-0.36 mm) thick. Unfortunately, mechanical stacking of layers of ferromagnetic materials is time consuming. Also, and more importantly, a substantial reduction in the magnetic properties of stacked cores as compared to continuous ribbon-wound cores can result due to the presence of joints in stacked cores. Non-uniform thickness of stacked layers can also increase stresses at the joints, thereby deteriorating magnetic performance. These drawbacks are even more pronounced in cores produced by stacking individual metallic glass ribbons because of the larger number of layers of metallic glass ribbon, as compared to silicon steel plates, needed to produce a stacked core of size suitable, for example, for use as a distribution transformer core.
Stacked magnetic cores are usually formed by arranging plates of ferromagnetic material in partially overlapping relationship to produce a closed loop corelet. A plurality of layers of such closed loops are arranged on top of one another to produce the stacked core. The stack is then placed under compression by clamping the layers to ensure dimensional stability and in an attempt to produce essentially no air gap between adjacent layers. The goal, of course, is to produce a dimensionally stable magnetic core have a packing factor approaching one (i.e., the actual density of the compacted product approaches the theoretical density of a single piece structure of the same material and dimensions).
Recently, the more pronounced problems associated with stacking single layers of metallic glass ribbon have largely been overcome as a result of the process disclosed in U.S. Pat. No. 4,529,457 and U.S. Pat. No. 4,529,458. According to the disclosure therein, a compact laminated structure composed of a plurality of layers of amorphous metallic ribbon is formed by holding a stack of ribbons at a pressure of at least 1,000 psi (6895 kPa) at a temperature between about 70 and 90% of the crystallization temperatures of the ribbons for a time sufficient to bond the ribbons. As a result, laminated product produced by this process overcome the time consuming task of stacking individual ribbons and reduce the problem associated with air gaps between successive layers.
Unfortunately, the use of amorphous metallic laminates as plates in the manufacture of stacked magnetic cores presents an additional significant problem. The laminates available today are generally of non-uniform shape because of slight variations in the dimensions of the strips employed to make the laminate. As a result of this non-uniformity, bending stresses are induced in the laminates when compressed to stabilize the core dimensions and to eliminate air gaps between stacked laminates. Bending stresses degrade the magnetic properties of ferromagnetic glassy ribbons used in transformer core manufacturing, particularly core loss properties, and yield magnetic cores with higher losses.
Accordingly, there remains a need in the art to produce stacked magnetic cores from metallic glass laminates without inducing unacceptably high bending stresses in the laminates.